Contemporary television receivers are required to operate in complex environments. For example analog (NTSC/PAL/Secam) and digital (ATSC/DVB/ISDB) television (DTV) signals now coexist within a crowded signal spectrum. The analog and DTV signals have different characteristics and place different requirements on television receivers. For example, the signal level (normally expressed as signal-to-noise ratio or SNR) required to demodulate a tuned DTV signal is much lower than that required to demodulate a tuned analog signal. In addition television receivers are often required to discriminate weak signals from geographically distant transmitters, while in the presence of spectrally near channels such as adjacent channels from nearby transmitters. The different characteristics of the analog and digital signals and various signal levels make it difficult to design receivers that are capable of tuning all channels with acceptable results.
To ensure proper output signal levels, television receivers commonly use automatic gain control (AGC). AGC is a mechanism that automatically adjusts the gain of a circuit such as a tuner in response to differences in received signal level to ensure that acceptable levels are available for subsequent processing.
Known television receiver AGC circuits attenuate the power in an input radio frequency (RF) signal in proportion to the power detected in an intermediate frequency (IF) tuned signal. This AGC technique, however, fails to account for power in signal spectra that fall outside of any filtering that may occur prior to the power detector and thus may cause components such as amplifiers, mixers, and filters to distort. Thus known AGC techniques properly control the gain of the tuned signal but may fail to properly manage the gain due to other signals.
What is needed, then, is a receiver such as a television receiver that performs AGC more flexibly in today's complex spectral environments while keeping distortion low.